Fineness of grind testing is a fundamental quality-control measurement that evaluates the degree of pigment dispersion in printing inks and directly affects print quality, color strength, gloss, and press runnability. This article explains the technical principles, applicable international and national standards such as ISO 1524, ASTM D1316, ASTM D1210, and GB/T 13217.3, typical sample types, step-by-step test procedures, and practical applications of ink fineness determination for printed products.
Technical Overview
Fineness of grind, also referred to as dispersion fineness or grind fineness, is a quantitative measure of the size of the largest pigment particles or pigment agglomerates present in a printing ink after the dispersion process. It is one of the most important routine indicators of ink quality, because incompletely dispersed pigment cannot deliver full color development, gloss, transparency, or rheological stability, and oversized particles can cause serious press problems and visible print defects.
Definition and Significance
In practical terms, the fineness of grind value represents the limiting particle size, expressed in micrometers (µm), beyond which pigment agglomerates are considered excessive for the intended printing process. A coarse grind can lead to streaks and haze in offset printing, clogged cells on gravure cylinders, blocked anilox rolls in flexography, mesh obstruction in screen printing, accelerated wear of plates and blades, and inconsistent color strength between batches. Conversely, achieving the specified fineness ensures uniform ink film formation, sharp dot reproduction, and stable ink transfer.
Measurement Principle
The test is performed with a precision instrument called a grindometer, grind gauge, or Hegman-type gauge. The gauge consists of a hardened steel block containing one or two wedge-shaped grooves whose depth decreases linearly from a defined maximum at one end to zero at the other end. A small amount of ink is placed in the deep end of the groove and drawn down with a calibrated scraper blade. Particles that are larger than the local groove depth protrude above the surface and appear as visible specks, scratches, or streaks. The position along the groove where these particles first appear in a continuous pattern corresponds to the maximum particle size in the sample, which is read directly against the engraved scale.
Relevant Standards
- ISO 1524 — Paints, varnishes and printing inks: Determination of fineness of grind.
- ASTM D1316 — Standard Test Method for Fineness of Grind of Printing Inks by the NPIRI Grindometer.
- ASTM D1210 — Standard Test Method for Fineness of Dispersion of Pigment-Vehicle Systems by Hegman-Type Gauge.
- GB/T 13217.3 — Chinese national standard test method for fineness of grind of printing inks.
Testing Samples
Fineness of grind determination is applicable to virtually all liquid paste or fluid printing inks as well as intermediate pigment dispersions. Sample handling requirements differ slightly depending on the ink system.
Offset and Lithographic Inks
Sheetfed, heatset web, and coldset offset inks are high-viscosity paste systems typically dispersed on three-roll mills. Typical acceptance limits are 15 µm to 20 µm or finer, since coarse particles directly affect plate wear and print sharpness on blanket cylinders.
Gravure and Flexographic Inks
Solvent-based and water-based liquid inks for publication gravure, packaging gravure, and flexography must pass through shallow engraved cells or fine anilox volumes, so fineness requirements are stricter, commonly 10 µm or less, with high-end packaging grades specified at 5 µm to 8 µm.
Screen Printing and Specialty Inks
Screen inks must pass through the mesh openings, so the specified fineness is usually related to the mesh count, typically around 20 µm to 35 µm. Metallic, conductive, and ceramic inks may carry special coarser limits due to intentionally large effect pigments.
UV-Curable Inks and Pigment Concentrates
UV offset and UV flexo inks, pigment dispersions, flushing pastes, and color concentrates used in ink formulation are also routinely tested to verify the dispersion endpoint of bead mills, three-roll mills, or high-shear dispersers.
Testing Items
The core deliverable of the test is the fineness of grind value, supported by several related observations that describe the dispersion state of the sample.
- Maximum particle size (fineness of grind), reported in micrometers or in NPIRI units on a 0 to 25 µm gauge.
- Dispersion uniformity, describing whether particles appear gradually along the groove or abruptly at one point.
- Presence of oversized agglomerates, gels, skins, fibers, or foreign contaminants that appear as isolated defects in the drawdown.
- Conformity assessment against the product specification or purchasing agreement, reported as pass or fail with the measured value.
Testing Methods
The internationally accepted procedures follow ISO 1524 and ASTM D1316, with GB/T 13217.3 applied for compliance in the Chinese market. The methods share the same physical principle and differ mainly in gauge geometry, scale units, and reading conventions.
Apparatus
A single-channel or dual-channel grindometer made of hardened, corrosion-resistant steel is used, with a measuring range selected to be only slightly greater than the expected fineness, for example 0 to 50 µm, 0 to 25 µm, 0 to 15 µm, or 0 to 10 µm. The NPIRI grindometer specified in ASTM D1316 has a 25 µm channel graduated in ten units, where one NPIRI unit equals 2.5 µm. A straight scraper with two polished edges and a blade longer than the groove width is used to perform the drawdown. The gauge must be periodically verified against certified reference depths because groove wear leads to erroneously fine readings.
Sample Conditioning
The ink sample is conditioned to the standard laboratory atmosphere, typically 23 °C ± 2 °C, and must be homogeneous and free of surface skins and entrained air. Any skin or contamination is removed without milling the sample, as additional mechanical work would alter the true grind state.
Test Procedure
- Clean the gauge and scraper with a suitable solvent and dry them thoroughly so the surface is completely free of residue, lint, and moisture.
- Place a sufficient quantity of the homogenized sample at the deep end of the groove, avoiding air bubbles.
- Hold the scraper perpendicular to the gauge surface with both edges in full contact, and draw the ink toward the shallow (zero) end at a uniform, moderate speed under steady pressure, completing the stroke within a few seconds.
- Within a few seconds after drawdown, before the film dries or flows back, view the groove surface obliquely at an angle to grazing light and identify the point where five to ten individual specks first appear across the groove width.
- Read the corresponding depth value from the engraved scale and record it.
- Repeat the determination at least three times on fresh portions of the sample and report the median value.
Expression of Results
Results are reported in micrometers, together with the gauge range used and the reading convention applied. Where required by specification, results may also be expressed in NPIRI units for printing inks or in Hegman units for pigment-vehicle systems, noting that these scales are inverted, with higher Hegman values indicating finer grind. The test report should identify the standard followed, the conditioning atmosphere, the number of determinations, and any observed abnormality such as gel particles or contamination.
Applications
Fineness of grind determination serves multiple functions across the printing and ink manufacturing value chain.
- Incoming quality inspection of inks and pigment dispersions at printing plants before release to production.
- Dispersion endpoint control during bead milling, three-roll milling, and high-shear dispersing in ink manufacturing.
- Research and development support for formulation optimization, pigment substitution, and resin carrier evaluation.
- Troubleshooting of print defects such as streaking, missing dots, hazy solids, and inconsistent color strength by identifying coarse particles as the root cause.
- Batch-to-batch consistency verification and quality documentation for packaging printers, label converters, and publication printers.
- Independent verification testing at accredited third-party laboratories for supplier qualification, customer disputes, and regulatory documentation.
FAQ
What does the fineness of grind value actually represent?
It represents the size of the largest pigment particles or agglomerates in the ink, not the average particle size. It is a screening measurement of dispersion completeness: a fine reading confirms that no oversized agglomerates remain, while the full particle size distribution must be measured by laser diffraction or microscopy if detailed information is required.
Which gauge should be selected for a given ink?
The gauge range should be only slightly larger than the expected fineness. A 0 to 25 µm NPIRI gauge is standard for most printing inks, while 0 to 15 µm or 0 to 10 µm gauges are used for high-quality gravure and flexographic systems, and a 0 to 50 µm gauge is reserved for coarse screen inks or pigment vehicles.
What fineness values are typically specified?
Offset inks are commonly specified at 15 µm to 20 µm or finer, gravure and flexographic inks at 10 µm or finer, and premium packaging inks at 5 µm to 8 µm. Screen printing inks may be accepted at 20 µm to 35 µm depending on the mesh. The exact limit is always agreed between the ink supplier and the printer based on the process requirements.
How quickly can the test be performed?
The measurement itself takes only a few minutes and requires only a few grams of sample, which makes it ideal as an at-press or in-line laboratory control test. Including conditioning and repeat determinations, a complete result is normally available within fifteen minutes.
What are the main sources of error?
Common errors include using a worn or uncalibrated gauge, drawing the scraper too quickly so that particles are dragged forward, reading after the film has started to dry, using a gauge range much larger than the actual fineness, and applying non-uniform scraper pressure. Strict adherence to the standard procedure and regular gauge verification minimize these effects.